Final answer:
The Earth began to warm about 14,000 years ago following the last ice age, as indicated by increases in O18, CO2, and methane content in ice layers. Ice cores like those from Vostok station provide evidence for past atmospheric CO2 levels and temperatures. These changes are associated with the greenhouse effect and warming Earth's climate.
Step-by-step explanation:
About 14,000 years ago, the Earth began to warm according to changes seen in annual ice layers in glaciers—the O18, CO2, and methane content in ice increased. This marked the end of the last ice age and the beginning of the current inter-glacial period. During the great ice ages, the temperature of the Northern Hemisphere periodically lowered with the last one lasting about 20,000 years. Ice cores, like those from the Russian Vostok station in East Antarctica, reveal past atmospheric CO2 levels and temperatures, providing evidence for these climatic changes.
Ice ages have caused significant changes in the Earth's climate. Carbon dioxide levels in the atmosphere have cycled between 180 and 300 parts per million (ppm), correlating closely with the planet's temperature fluctuations. The changes in the levels of CO2, methane, and O18 trapped within ice layers, have been identified through methods like analyzing the amount of CO2 and deuterium (a nonradioactive isotope of hydrogen) present in ice cores.
As the Earth warmed after the last ice age, changes in the composition of the ice layers were documented, which included an increase in the trapped gases — indicative of a higher greenhouse effect and a warmer climate. This warming phase caused large ice sheets to melt, signaling the transition to warmer temperatures.